Efficiently separating solids from liquids is a widespread technical problem. One of the most practical and robust methods of achieving this remains the use of a sieve, or screen, to sift the solids from the mixture of liquid and solid.
When drilling for oil and/or gas, synthetic drilling fluids, or muds, are used. As these muds are relatively expensive to manufacture, once used they are typically recovered in a process including sifting rock, shale and other debris from the mud. This involves the use of a so-called shaker which has fitted, one or more sifting screens, made up of a screen frame with one or more sheets of woven wire mesh, or screen, stretched over and secured to it. In use, the shaker vibrates the sifting screen or screens, to aid the sifting process.
In order for such sifting screens to be able to withstand the rigours of such a process, they must have a certain rigidity and be very hard-wearing. This has resulted in a design of sifting screen having a screen frame which has a plurality of reinforcing “ribs”. A common design of screen frame is rectangular comprising an outer rectangular perimeter with each side connected to its opposing side by a plurality of ribs. Such a design results in a plurality of rectangular openings. Typically the screen is attached not only to the rectangular perimeter but also to the ribs, to provide better adhesion of the screen to the frame and prolonging its lifetime.
In use the long sides of the perimeter are often clamped in position, leaving the short sides unclamped. The direction of bulk flow of the liquid/solid mixture is substantially parallel to the clamped sides.
The rectangular openings are arranged such that the long dimension of the rectangular openings is substantially parallel to the direction of bulk flow of the liquid/solid mixture passing over the screen. This is because it is believed that the solids will slow down when passing over a rib and so the fewer transverse ribs there are to pass over the less erratic will be the motion of the solids. This has the effect that the number of plastics ribs per unit length extending between clamped sides is less than the number of plastics ribs per unit length extending between unclamped sides.
In view of the fact that sifting screens are man-handled into position, such screen frames have for some time been made from plastics material to reduce weight. A common design of plastics screen frame is reinforced by including a metal wire structure, embedded within the plastics rectangular perimeter and rib arrangement.
However, despite the measures taken to provide sufficient rigidity, the present inventors have found that vibratory motion typically involved in shakers is not successfully transmitted by the screen frame to the attached screen. Excessive motion of screens has been observed, known as “whipping”, which can result in erratic solids conveyancing and premature screen failure.